The structures and mechanisms of assembly of macromolecules and marcromolecular complexes are studied, primarily by high resolution electron microscopy complemented with computer image processing. Over the past year, our major efforts have been devoted to analysis of (i) the tail-fibers of bacteriophage T7; and (ii) frozen-hydrated transverse thin sections of skeletal muscle in the relaxed and rigor states. (i) The T7 tail-fiber represents an experimentally tractable example of a viral receptor-recognition protein. We have combined piece-wise correlation averaging of electron micrographs of molecules prepared both by negative staining and by freeze-drying without staining, with predictions based on the known sequence of the gp17 protein (553 residues), to formulate a detailed model of this fibrous protein. The fiber is a trimer a consists of three domains. The proximal and distal half-fibers are both rods approximately 16nm long, joined at a kink-angle of approximately 90 degrees and are linked to the virion by the third domain. Of particular interest is the proximal half-fiber which is a triple-stranded coiled-coil of alpha-helices, one of the few well-authenticated examples of this conformation. (ii) We have developed and applied methods for the analysis of frozen-hydrated transverse thin sections of muscle. Such images combine the advantages of good preservation of native structure (unlike conventional thin sections) and phase determinacy (unlike X-ray fiber diffraction). The results reveal differences between the distributions of density in muscle in the relaxed and rigor states.